Many aspects of semiconductor processing have been automated for efficiency and for better control of cleanliness of the processes. This automation has been achieved in part by extensive employment of robots and other automatic manipulators and handling devices for moving semiconductor wafers and other objects to and from processing positions, for moving wafers and other objects from one processing apparatus to another, and for positioning wafers and other objects as required for performing a particular process step.
An exemplary process used extensively in the manufacture of semiconductor devices, especially of integrated circuits, is “microlithography.” Microlithography involves the “transfer” of a pattern, having extremely small features, from a pattern-defining object to an imprintable object. In “projection-microlithography” the pattern-defining object is usually termed a “reticle” or “mask” (generally termed “reticle” herein) and the imprintable object is termed a “substrate,” which usually is a semiconductor wafer that may or may not already have previously formed circuit layers on its surface. (Consequently, substrates are also generally termed “wafers.”) So as to be imprintable with an image, the substrate is coated with a radiation-sensitive composition termed a “resist.”
Projection-microlithography systems are used extensively, for example, for manufacturing integrated circuits, microprocessors, memory “chips,” and the like. These products characteristically comprise multiple functional layers of microscopic circuit elements, all interconnected together in 3-dimensional space. Typically, microlithography is used for patterning most, if not all, the functional layers. In each microlithographic step, the pattern-defining object (usually the reticle) defines the respective pattern for the particular functional layer to be formed. A beam of exposure radiation, termed an “illumination beam,” is directed by an “illumination-optical system” from a source to the pattern-defining object. Interaction of the illumination beam with the pattern-defining object (i.e., selective transmission of the illumination beam through the pattern-defining object or selective reflection of the illumination beam from the pattern-defining object) results in patterning of the beam (now termed a “patterned beam” or “imaging beam”), which renders the patterned beam capable of forming an aerial image of the illuminated pattern. The patterned beam is projected by a “projection-optical system” onto a desired location on the resist-coated substrate where an actual image of the illuminated pattern is formed. Thus, a projection-microlithography system is a type of camera that projects and forms an image on the resist-coated substrate (analogous to a sheet of photographic paper) corresponding to the pattern defined by the pattern-defining object (analogous to a photographic negative, for example).
For exposure, the reticle usually is held by a respective chuck on a device called a “reticle stage,” and the substrate usually is held by a respective chuck on a device called a “substrate stage” or “wafer stage.” These stages also are typically equipped to perform highly accurate positional measurements and positioning in response to the measurements. Some microlithography systems have multiple reticle stages and/or multiple substrate stages which allow, for example, pre-exposure or post-exposure manipulations to be performed on other reticles and substrates, respectively, as an exposure is being performed on a particular substrate.
Before being exposed, and to prepare the substrate for exposure, the substrate is usually primed and then coated with a layer of a suitable resist. Before actual exposure, the resist usually is treated such as by a soft-bake step (“pre-exposure bake”). After exposure, the substrate may be soft-baked again (“post-exposure bake”), followed by development of the resist and a hard-bake step to prepare the resist for downstream process steps such as etching, doping, metallization, oxidation, or other suitable step in which the remaining resist on the substrate surface serves as a process mask. Thus, the respective layer is formed on the substrate. As noted above, multiple layers must be formed on the substrate in order to fabricate actual semiconductor devices, so these or similar process steps usually need to be repeated multiple times during the fabrication of the devices. During formation of each layer, steps must be taken to ensure proper and accurate registration of the new layer with the previously formed layer(s).
The substrate usually is sufficiently large to allow formation of multiple semiconductor devices at respective locations (“dies”) on the substrate. Exposure of multiple dies on the substrate can be die-to-die in one shot per die (characteristic of a “step-and-repeat” exposure scheme) or by scanning each die (characteristic of a “step-and-scan” exposure scheme). In step-and-scan each die typically is exposed by scanning in a scanning direction, wherein both the reticle and the substrate are moved during scanning. Movement of the reticle and substrate can be in the same direction or in opposite directions. If the projection-optical system has a magnification factor (M) other than unity, then the scanning velocity of the substrate typically is M times the scanning velocity of the reticle.
After completing the fabrication of all the required layers on the surface of the substrate, the dies are cut one from the other. Individual dies are mounted on a packaging substrate, connected to pins or the like, and encased to form finished semiconductor devices. The finished devices typically undergo rigorous testing before being released for sale.
Accompanying the acknowledgement of an apparent limit (not yet defined) of the minimum feature size of a pattern that can be transferred at an acceptable resolution by optical microlithography, a substantial ongoing effort currently is being directed to the development of a practical “next-generation lithography” (“NGL”) technology. One promising NGL approach is EUV lithography (“EUVL”) performed generally in the wavelength range of 5-20 nm and more specifically at a wavelength in the range of approximately 11-14 nm.
One challenge posed by EUVL is the substantial scattering and attenuation of an EUV beam by normal-pressure air. Consequently, the propagation path of an EUV beam in an EUVL system must be maintained at high vacuum. Another challenge posed by EUVL is the lack of any known material that is EUV-transmissive and capable of refracting EUV light. Consequently, all the optical elements in an EUV optical system must be reflective rather than refractive. These reflective optical systems and elements include the illumination-optical system, the projection-optical system, and the reticle itself.
Certain aspects of a conventional EUVL system 110 are shown in FIG. 4. The depicted system includes an EJV source 112, an illumination-optical system 114, and a projection-optical system 116. The EUV source 112 produces pulses of EUV light from, for example, a laser-induced plasma or electrical-discharge-induced plasma.
The depicted illumination-optical system 114 includes a collimator mirror 118, a first fly-eye mirror 120, a second fly-eye mirror 122, a first condenser mirror 124, a second condenser mirror 126, and a grazing-incidence mirror 128. These mirrors are mounted at respective locations on a rigid frame (not detailed) so as to place the mirrors in proper respective positions relative to each other. The collimator mirror 118 collimates the EUV light from the source 112 as the EUV light reflects from the collimator mirror. The collimated light propagates to the first fly-eye mirror 120, from which the light reflects to the second fly-eye mirror 122. The fly-eye mirrors 120, 122 make the illumination intensity of the EUV light substantially uniform over the illumination field. From the second fly-eye mirror 122 the EUV light assumes a gradually convergent characteristic as the EUV light propagates to and reflects from the first and second condenser mirrors 124, 126. From the second condenser mirror 126 the EUV light reflects (at grazing incidence) from the grazing-incidence mirror 128 (usually a planar mirror) to the reticle 130 where the illumination field illuminates respective selected portions of the reticle pattern at particular instances in time. During illumination the reticle 130 is mounted (reflective-side facing downward) on a reticle chuck 132 that is mounted on a movable reticle stage 134. The reticle stage 134 positions the reticle 130 in three-dimensional space as required for illumination of the desired portions of the reticle pattern by the illumination field at respective instances in time.
The particular type of illumination-optical system 114 shown in FIG. 4 is a 6-mirror system. So as to be reflective to incident EUV light at less than grazing angles of incidence, the collimator mirror 118, fly-eye mirrors 120, 122, and condenser mirrors 124. 126 have surficial multilayer-interference coatings (e.g., multiple superposed and very thin layer pairs of Mo and Si) that render the surfaces of these mirrors reflective to incident EUV light. Due to the manner in which the EUV light reflects from the grazing-incidence mirror 128 (i.e., at grazing angles of incidence), the grazing-incidence mirror need not have a multilayer coating. In the EUV source 112, the collector mirror 136 also has a multilayer-interference coating.
Because of the lack of suitable reticle-making materials exhibiting significant transparency to EUV light, the reticle 130 is a reflective reticle. EUV light from the grazing-incidence mirror 128 is incident on the reticle 130 at a small angle of incidence (approximately 5 degrees). So as to be reflective to EUV light at such a small angle of incidence, the reticle 130 also has a multilayer-interference coating as well as EUV-absorbent bodies that define, along with spaces between the bodies, the particular pattern on the reticle that is to be transferred to the substrate. Thus, as the EUV light reflects from the irradiated region of the reticle 130, the EUV light is “patterned” by differential reflection of the light from the pattern defined on the reticle. The patterned beam 138 acquires an aerial image of the pattern on the reticle 130 and thus is rendered capable of imaging the illuminated pattern on the surface of the resist-coated substrate 140.
To form the image on the surface of the substrate 140, the “patterned” EUV light 138 reflected from the reticle 130 passes through the projection-optical system 116, which also contains multiple reflective mirrors (not detailed), to the resist-coated substrate (wafer) 140. During exposure the wafer 140 is mounted (face up) to a wafer chuck 142 that is mounted on a wafer stage 144.
When performing microlithography, it is necessary to move wafers into and out of position for imaging, while maintaining an extremely high standard of wafer cleanliness. To such end, “wafer handlers” (wafer-manipulating robots) 148 are extensively used for placing wafers 140 on the wafer stage 144 for exposure and for removing wafers from the wafer stage after exposure. The wafer stage 144 is movable to place the wafer 140 in the proper positions for exposure of different regions of the wafer. Similarly, the reticle stage 134 is movable to allow placement of the reticle 130 at proper positions for making exposures. Since the reticle 130 is very valuable and also must be kept extremely clean, the reticle normally is moved to and from the reticle stage 134 using a “reticle handler” (reticle-manipulating robot) 146, which avoids contaminating the reticle by human handling and offers high accuracy and precision of reticle placement.
As noted above, in current EUV microlithography systems, the reticle 130 is held face-down on the reticle stage 134 during use for making lithographic exposures. To hold the reticle 130 in such a manner, the reticle stage 134 is equipped with a “reticle chuck” 132 that must be capable of holding the mass of the reticle securely against the force of gravity and also against acceleration and deceleration forces imparted to the reticle 130 during motions of the reticle stage 134. Since EUV microlithography must be performed under high-vacuum conditions, the reticle chuck 132 must be capable of performing its task reliably under high-vacuum conditions. Several types of reticle chucks exhibit this kind of performance, but the currently best approach for holding the reticle 130 is attraction of the reticle to the surface of the reticle chuck 132 by electrostatic or Lorentz forces. In both of these approaches the reticle chuck 132 includes electrodes that are electrically energized whenever the chuck must hold the reticle 130, and that are de-energized whenever the chuck is not holding the reticle. In either of these approaches, the reticle must be in a proper position and in close proximity to the surface of the reticle chuck 132 at commencement of electrode energization to provide conditions under which the reticle 130 can be fully held by the chuck. If the reticle 130 is not fully held, then the reticle either will not adhere or will only partially adhere to the chuck 132 as the reticle handler 146 (which has delivered the reticle to the chuck) is retracted or will fall from the chuck 132 as or after the reticle handler 146 is retracted.
Thus, whenever a reticle handler 146 has delivered the reticle 130 to the reticle chuck 132, it is desirable that a determination be made as to whether the reticle is properly placed relative to the chuck and is being held securely by the chuck before the reticle handler 146 is retracted (i.e., moved away from the reticle chuck 132). Conventionally, this determination is made using extraneous reticle-position sensors, e.g., sensors that detect light reflected from the reticle 130 or sensors that detect a break in a beam of light caused by the reticle being at the reticle chuck 132. A key drawback of these types of sensors is that they add further hardware and more complexity to the microlithography system in general. For example, these types of sensors require additional electronics to control their operation, to process signals from them, and to generate and actuate responses to their signals.